the carbonyl group

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In aldehydes the carbonyl group is at the end of the carbon chain and so has at least one hydrogen attached to it.

The carbonyl group (>C=O) is the functional group found in compounds such as aldehydes, ketones, and carboxylic acids.

In ketones the carbonyl group is in the middle of a carbon chain and so has two alkyl groups attached to it.

The carbonyl group


Representing aldehydes and ketones

Aldehydes and ketones cannot be distinguished by their molecular formula because they have the same functional group.

Both have the molecular formula C3H6O

What is the molecular formula of the aldehyde and ketone below?

an aldehyde a ketone


Representing aldehydes and ketones

Aldehydes and ketones are therefore represented either using displayed or structural formulae.

CH3CHO CH3CH2CH2CHO

CH3COCH2CH3 CH3CH2COCH2CH3


Naming aldehydes

Aldehydes are named using the suffix –al, and follow the same conventions as for naming alkanes.

ethanal propanal

2-methylbutanal


Like alkenes, ketones with four or more carbon atoms display positional isomerism because the carbonyl group may appear between different carbon atoms. In these cases, a number is used before the –one to indicate the first carbon involved.

Naming ketones

Ketones are named using the suffix –one.

propanone hexan-3-one


Naming aldehydes and ketones activity


Synthesis of aldehydes and ketones


Properties of aldehydes and ketones

The carbonyl group is polar due to the greater electronegativity of oxygen (3.4) than carbon (2.6). This influences the properties of aldehydes and ketones, such as solubility.

δ+ δ-

Small aldehydes and ketones are soluble in water due to hydrogen bonding between a lone pair on the oxygen of the carbonyl group and the hydrogen of the water.

As size increases, solubility decreases due to interference in hydrogen bonding by the hydrocarbon ‘tails’ of the aldehydes/ketones.


Boiling points: a comparison


Boiling points and intermolecular forces

The general increase in boiling points from alkanes to aldehydes/ketones and alcohols is due to the intermolecular forces between each type of molecule.

Alkanes are only held together by van der Waals forces. These forces increase with the size/length of a molecule.

The polar carbonyl group in aldehydes and ketones means that as well as van der Waals forces, these molecules are also held together by dipole–dipole interactions.

Alcohols are held together by van der Waals forces and dipole–dipole interactions. In addition, these molecules can form hydrogen bonds with each other, due to the slightly positive hydrogen atom of the hydroxyl group.


Reactivity of the carbonyl group

Some of the chemical properties of aldehydes and ketones result from the polar nature of the C=O bond.

The positive charge on the carbon atom makes it open to attack by nucleophiles.

Although double bonds require more energy to break than single bonds, compounds with double bonds tend to be more reactive as addition reactions are possible.

Aldehydes and ketones can be reduced, forming primary and secondary alcohols respectively.

Aldehydes may also be oxidized to carboxylic acids.

δ+ δ-


Nucleophilic addition using cyanide


Dangers of HCN

Hydrogen cyanide (HCN) is highly volatile liquid (boiling point 26 °C), which has a faint bitter almond smell.

In solution, hydrogen cyanide partially dissociates:

A safer alternative is potassium cyanide, which is a solid at room temperature and is therefore easier to handle. An acidified solution contains both the H+ and CN- ions.

Hydrogen cyanide is highly toxic because it inhibits a mitochondrial enzyme that is essential for respiration. Being so volatile and flammable, it is difficult to handle safely.

HCN(aq) H+(aq) + CN-

(aq)


Reduction of aldehydes and ketones


Distinguishing aldehydes & ketones


Detecting a carbonyl group


Reactions of aldehydes and ketones


Aldehydes and ketones: true or false?


Carboxylic acids

Carboxylic acids have a carboxyl group(-COOH) consisting of a carbonyl group and a hydroxyl group attached to the terminal carbonyl carbon.

Carboxylic acids are named using the suffix –oic acid.

Ethanoic acid is the acid that gives vinegar its sharp taste and smell. It is also important in the chemical industry and about 6.5 million tonnes are used worldwide each year

Methanoic acid is the simplest carboxylic acid and is found in bee and ant stings.


Naming carboxylic acids


Boiling points of carboxylic acids


Synthesis of carboxylic acids


Carboxylic acids can be created by the oxidation of aldehydes by oxidizing agents such as potassium dichromate(VI).

They can also be created by the oxidation of primary alcohols, again using potassium dichromate(VI).

Oxidation of 1° alcohols and aldehydes


Carboxylic acids can also be prepared by the hydrolysis of nitriles.

Hydrolysis of nitriles

The nitrile is refluxed with water and hydrochloric acid. The carboxylic acid can then be distilled from the reaction mixture.


Reactivity of carboxylic acids

The reactivity of carboxylic acids results, in part, from the polarization of its bonds.

The reactions of carboxylic acids include:

neutralisation – the carboxylic acid loses a proton to form a carboxylate salt

nucleophilic substitution – the positively-charged carbon is attacked by a nucleophile, resulting in substitution of the OH group

esterification – reaction with an alcohol to form an ester.

δ+

δ-

δ-

δ+


Forming the carboxylate ion

Carboxylic acids are weak acids and partially dissociate in aqueous solution:

carboxylate ion

The negative charge is delocalized across the carboxylate group, resulting in a more stable ion. The delocalization is represented by adding a second dotted line to the carbon oxygen bonds, which are both equivalent.


Reactions with carbonates

Carboxylic acids are the only organic compounds that are strong enough acids to react with either sodium carbonate or sodium hydrogencarbonate.

Reaction of ethanoic acid and sodium carbonate:

2CH3COOH + Na2CO3 → 2CH3COONa + CO2 + H2O

sodium ethanoate

Reaction of methanoic acid and sodium hydrogencarbonate:

HCOOH + NaHCO3 → HCOONa + CO2 + H2O

sodium methanoate

The formation of bubbles of carbon dioxide gas makes these reactions useful as a test for carboxylic acids.


Reactions with alkalis

Carboxylic acids will react with alkalis, such as sodium hydroxide, in a neutralization reaction:

RCOOH + NaOH → RCOONa + H2O

Reaction of propanoic acid and sodium hydroxide:

Reaction of butanoic acid and potassium hydroxide:

CH3CH2COOH + NaOH → CH3CH2COONa + H2O

sodium propanoate

CH3CH2CH2COOH + KOH → CH3CH2CH2COOK + H2O

potassium butanoate


Carboxylic acids: a summary


Glossary


What’s the keyword?


Multiple-choice quiz

the carbonyl group

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